Viruses are intracellular parasites that require the biochemical machinery of a host cell for replication and propagation. All virus particles contain some genetic information that encodes viral structural proteins and enzymes. The genetic material may be DNA or RNA, in double- or single stranded form. (Virology, Fields ed., third edition, Lippencott-Raven publishers, pp 72–83 (1996)). The viral nucleic acid is surrounded by a coat of proteins called the capsid. (Id.) In some viruses the capsid is surrounded by an additional layer comprised of a lipid membrane, referred to as the envelope. (Id. at 83–95).
The typical viral life cycle begins with infection of a host cell through attachment of the virus particle to a cell surface receptor and internalization of the viral capsid. (Id. at 103). Accordingly, a virus' host range is limited to cells that express an appropriate cell surface receptor. Once internalized, the virus particle is disassembled and its nucleic acid is transcribed, translated or replicated. (Id.) At this point, the virus may undergo lytic replication, where new virus particles are formed and released from the infected cell. (Id. at 105–11). The Influenza virus is a typical example of a virus that undergoes lytic replication immediately upon infection of a host cell. (Id. at 1369–85).
Alternatively, a virus may enter a latent phase, referred to as lysogeny, where the genome is replicated but few if any viral proteins are actually expressed and viral particles are not formed. (Id. at 219–29). Herpesviruses such as the Epstein-Barr Virus are typical examples of viruses that establish latent infection in the host cells. (Id. at 229–34). Eventually, in order for the virus to spread, it must exit lysogeny and enter the lytic phase. The viral particles that are released during the lytic phase infect other cells of the same individual or can be transmitted to another individual where a new infection is established.
Since the viral life cycle comprises both an intracellular and extracellular phase, both the humoral and cell-mediated immune defense systems are important for combating viral infections. (Id. at 467–73). Antibodies directed against viral proteins may block the virus particle's interaction with its cellular receptor or otherwise interfere with the internalization or release processes. (Id. at 471). An antibody capable of interfering with the viral life cycle is referred to as a neutralizing antibody.
During intracellular replication, viral proteins, which are foreign to the host cell, are produced and some of these proteins are digested by cellular proteases after coupling to a Major Histocompatibility Complex (MHC) molecule presented on the surface of the infected cell. (Id. at 350–58). Thus, the infected cell is recognized by T-lymphocytes, macrophages or NK-cells and killed before the virus replicates and spreads to adjacent cells. (Id. at 468–70). In addition, the presence of viral nucleic acids, most notably as double-stranded RNA, triggers the infected cell to shut down its translation machinery and to produce antiviral signaling molecules known as interferons. (Id. at 376–79).
Viruses have evolved various means of evading the immune defense system of the host, however. By establishing latency (i.e., lysogeny), for example, the virus does not enter the lytic phase and avoids the humoral immune defense system. (Id. at 224). During the latent phase, few viral proteins are produced and infected cells have only a minimal ability to present evidence to surrounding lymphocytes and macrophages of their infected state. (Id. at 225–26). Additionally, some viral proteins, most notably those produced during latency, evolve polypeptide sequences that cannot be efficiently presented to the cell mediated immune defense system. (Levitskaya el al., Nature 375:685–88 (1995)). Finally, some viruses may actively interfere with the immune response of the infected host, for instance by preventing surface expression of MHC molecules (Fruh et al., J. Mol. Med. 75:18–27 (1997)), or by disrupting interferon signaling (Fortunato et al., Trends Microbiol. 8:111–19 (2000)).
Particularly evasive are the hepatitis viruses, which are not classified as a family but are grouped based on their ability to infect cells of the liver. Hepatitis C Virus (HCV) belongs to the Flaviviridae family of single-stranded RNA viruses. (Virology, supra, pp 945–51). The HCV genome is approximately 9.6 kb in length, and encodes at least ten polypeptides. (Kato, Microb. Comp. Genomics, 5:129–151 (2000)). The genomic RNA is translated into one single polyprotein that is subsequently cleaved by viral and cellular proteases to yield the functional polypeptides. (Id.) The polyprotein is cleaved to three structural proteins (core protein, E1 and E2), to p7 of unknown function, and to six non-structural (NS) proteins (NS2, NS3, NS4A/B, NS5A/B). (Id.) NS3 encodes a serine protease that is responsible for some of the proteolytic events required for virus maturation (Kwong et al., Antiviral Res., 41:67–84 (1999)) and NS4A acts as a co-factor for the NS3 protease. (Id.) NS3 further displays NTPase activity, and possesses RNA helicase activity in vitro. (Kwong et al., Curr. Top. Microbiol. Immunol., 242:171–96 (2000)).
HCV infection typically progresses from an acute to a chronic phase. (Virology, supra, pp 1041–47). Acute infection is characterized by high viral replication and high viral load in liver tissue and peripheral blood. (Id. at 1041–42.) The acute infection is cleared by the patient's immune defense system in roughly 15% of the infected individuals; in the other 85% the virus establishes a chronic, persistent infection. (Lawrence, Adv. Intern. Med., 45:65–105 (2000)). During the chronic phase replication takes place in the liver, and some virus can be detected in peripheral blood. (Virology, supra, pp 1042).
Essential to the establishment of a persistent infection is the evolution of strategies for evading the host's immune defense system. HCV, as a single stranded RNA virus, displays a high mutation rate in the replication and transcription of its genome. (Id. at 1046). Thus, it has been noted that the antibodies produced during the lytic phase seldom neutralize virus strains produced during chronic infection. (Id.) Although it appears HCV is not interfering with antigen processing and presentation on MHC-I molecules, the viral NS5A protein may be involved in repression of interferon signaling through inhibition of the PKR protein kinase. (Tan et al., Virology, 284:1–12 (2001)).
The infected host mounts both a humoral and a cellular immune response against the HCV virus but in most cases the response fails to prevent establishment of the chronic disease. Following the acute phase, the infected patient produces antiviral antibodies including neutralizing antibodies to the envelope proteins E1 and E2. (Id. at 1045). This antibody response is sustained during chronic infection. (Id.) In chronically infected patients, the liver is also infiltrated by both CD8+ and CD4+ lymphocytes. (Id. at 1044–45). Additionally, infected patients produce interferons as an early response to the viral infection. (Id. at 1045). It is likely that the vigor of the initial immune response against the infection determines whether the virus will be cleared or whether the infection will progress to a chronic phase. (Pape et al., J. Viral. Hepat., 6 Supp. 1:36–40 (1999)). Despite the efforts of others, the need for efficient immunogens and medicaments for the prevention and treatment of HCV infection is manifest.